1Make and bake cake mix as directed on box for 13x9-inch pan, using water, oil and egg whites. Cool. Line cookie sheet with waxed paper. Crumble cake into large bowl. Add frosting; mix well. Shape into 1-inch squares; place on cookie sheet. Freeze until firm; keep refrigerated.
2In 3 separate bowls, stir each color melted candy and 2 teaspoons shortening until smooth. Dip tip of 14 of the lollipop sticks about 1/2 inch into melted pink candy and insert stick into 1 cake square no more than halfway. Repeat, dipping 14 sticks in yellow candy and 14 in blue. Return to cookie sheet. Refrigerate 20 minutes. Remove from refrigerator a few at a time. Dip each cake square into melted candy to cover (using same color to match stick); tap off excess. Poke opposite end of stick into foam block. Let stand until set.
3On 1 side of each square, pipe white icing to look like a block. Pipe letter A, B or C within each outline. Let stand until set.